docs: def set_contains(self, key, value):
          """
          Determine if an item exists in a set
          :param key: The document ID of the set
          :param value: The value to check for
          :return: True if `value` exists in the set
          :raise: :cb_exc:`NotFoundError` if the document does not exist
          """
          rv = self.get(key)
          return value in rv.value